Wellbores provided for the purpose of production of oil and gas are typically cased with steel casings, and cement is placed around the annulus of the steel casing. This cement is usually placed by pumping a cement slurry down the casing and out the bottom of the casing into the annulus between the wellbore and the outside of the casing. The slurry must therefore remain pumpable for an extended time period, but it is also desirable for the cement slurry to cure to a reasonable hardness within a short time period after the cement slurry is placed within the wellbore so that the well completion process may proceed.
Cements are available that are capable of withstanding elevated temperatures. Temperatures as high as 1500.degree. F. to 2500.degree. F. may be expected in production or injection wells when some thermal recovery processes are utilized. Provision of wellbore cements that can maintain sufficient strength at these temperatures and have acceptable slurry properties remains a problem.
U.S. Pat. No. 3,734,188 discloses wellbore cement compositions comprising alumina based cements and accelerators for setting those cement. These cements can withstand temperatures of at least 1600.degree. F. according to patent '188.
U.S. Pat. No. 5,226,961 discloses other high temperature cement compositions that are also based on alumina cements. Patent '961 discloses low density aggregates and conversion of drilling muds to alumina cement slurries. In particular, graphite as a low density aggregate that imparts excellent thermal conductivity to the cured cement is disclosed. Magnesium chloride is utilized in these compositions as a set retarder.
U.S. Pat. Nos. 3,180,748 and 3,605,898 address the problem of retarding the setting of cements. Patent '898 discloses heptalactone as a preferred retarding agent for a wide variety of cement slurries that includes alumina cement slurries. Patent '748 discloses boric acid, calcium lignin sulfonate and alkali metal or alkaline earth metal phosphates as retarders for alumina cement slurries. The phosphates are preferred as retarders when the cement slurries include silica flour as an aggregate. Cements disclosed in patent '748 are said to be capable of withstanding temperatures of 1500.degree. F. to 2000.degree. F., and examples are said to remain pumpable for up to three hours and twenty five minutes. Retarders for alumina cement slurries that are more effective than those available in the prior art would still be desirable.
It is therefore an object of the present invention to provide a cement slurry composition that, after being cured, is capable of withstanding service temperatures of 1500.degree. F. to 2500.degree. F. It is a further object to provide such a composition wherein the slurry remains pumpable for more than three hours after initially being blended.